1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and, more particularly, it relates to an image forming apparatus for forming an image by scanning a photosensitive member by laser beam.
2. Related Background Art
Among image forming apparatuses, an electrophotographic laser printer is known as a high speed and low noise printer. Such laser printer generally performs binary recording wherein an image such as a character, figure and the like is formed by illuminating or not illuminating a laser beam onto a photosensitive member. In general, since the recording of the character, figure and the like does not require an intermediate tone, the printer can be simplified.
Recently, there has been proposed an intermediate tone pixel forming method wherein high resolving power can be obtained without decreasing the recording density. In this method, the intermediate tone image is formed by modulating a pulse width of laser beam illumination in response to an image signal (PWM). According to this PWM system, an image can be formed with high resolving power and high gradient. Thus, this system is inevitable for color image forming apparatuses wherein high resolving power and high gradient are required. According to this PWM system, it is possible to effect the area gradation of a dot formed by a beam spot for each pixel, and, thus, the intermediate tone can be achieved without reducing the pixel density (recording density).
Even in the image forming apparatuses wherein the image is formed by scanning the photosensitive member by the laser beam, the improvement in the resolving power has been requested. It is considered that a diameter of the laser beam spot is decreased to increase the resolving power. However, in order to decrease the diameter of the laser beam spot, power of a focusing lens must be increased, with the result that the image forming apparatus itself becomes bulky and expensive. Further, the depth of focus is reduced, thereby making the focusing on the photosensitive member unstable.